This invention relates to voltage controlled amplifiers, and more particularly to a voltage controlled amplifier capable of operating with bipolar input signals, or capable of two quadrant, low noise operation.
Unipolar circuits capable of producing a logarithmic response are well known in the art, and are disclosed in the Gibbons U.S. Pat. No. 3,237,028.
The Gibbons U.S. Pat. No. 3,293,450 disclosed unipolar circuits capable of having anti-logarithmic response, and also discloses the combination of a unipolar circuit capable of logarithmic response and a unipolar circuit capable of anti-logarithmic response to perform certain mathematical functions, such as multiplication.
The Pearlman U.S. Pat. No. 3,329,836 discloses that it is old to provide a logarithmic amplifier circuit having double feedback loops to render it capable of handling bipolar input signals.
The Embley U.S. Pat. No. 3,532,868 discloses a log-antilog multiplier circuit capable of four quadrant multiplication, by introducing an offset voltage of sufficient magnitude to insure that the unipolar log-antilog elements remain in the unipolar domain to which the elements are log responsive. However, because the bias current or offset voltage must be greater than the signal current, then noise levels are greatly increased, and the input signals are limited to a range within the magnitude of the offset voltage.
The Blackmer U.S. Pat. No. 3,714,462 describes a bipolar multiplier circuit including an input amplifier having double feedback loops, each including a log transistor of opposite polarity from each other. Each log transistor is connected to an antilog transistor, the antilog transistors also being of opposite polarity or conductivity from each other. A voltage control signal is applied to one or more of each of the log transistors and the antilog transistors.
Although the bipolar multiplier circuit of the above Blackmer patent effectively eliminates the disadvantages of the multiplier circuit disclosed in the above Embley patent, nevertheless, the Blackmer circuit is affected by the differences in the parameters of transistors of different polarity or conductivity, which will cause the log-antilog elements to respond somewhat differently to positive signal wave forms than to negative signal wave forms. Accordingly, the Blackmer circuit would require very careful matching of transistor pairs, together with compensating circuitry to balance the inherent transistor differences and to obtain low distortion operation over a wide control range.
Furthermore, since present manufacturing processes do not permit the manufacture of monolithic pairs of transistors of opposite conductivity suitable for logarithmic amplifiers, matched pairs of discrete transistors are mandatory for successful operation of the Blackmer circuit. Moreover, the discrete transistors employed in the Blackmer circuit must be mounted on a common heat sink in order to maintain temperature equilibrium.